Magnetic Interaction Between Stars and Accretion Disks

Uzdensky, Dmitri

doi:10.1007/978-94-017-0491-5_41

Cited by 2 publications

(3 citation statements)

References 26 publications

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“…The disc–magnetosphere interaction basically transforms angular momentum (differential rotation) into toroidal plasmoids that are ejected from the system. Basically all models can be fitted into a basic configuration with a current sheet separating two distinct regions: an inner stellar outflow and an external disc flow (Uzdensky 2004). Magnetic flux dissipation is expected to be produced in the current layer leading to plasmoid ejections, as well as to the injection of high‐energy particles (cosmic rays) in the environment leading to generation of X‐rays and UV radiation (Gómez de Castro 2004).…”

Section: Discussionmentioning

confidence: 99%

On the source of dense outflows from T Tauri stars - II. Warm disc winds

Castro

Ferro-Fontán

2005

Monthly Notices of the Royal Astronomical Society

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Recent ultraviolet (UV) observations suggest that there is a hot (T e 80 000 K) and dense (Ne 10 10 cm −3 ) wind associated with the large-scale jets observed in classical T Tauri stars (cTTSs). The observations of these rather evolved sources cannot be fitted with the classical cold disc wind solutions. This is not unexpected since the accretion rates are moderate ( 10 −8 M yr −1 ) and the wind lighter than at earlier phases. Henceforth, X-ray radiation from the star and the star-disc interaction region is expected to modify the thermal structure of the inner disc, e.g. the base of the disc wind. Thus, thermal pressure is relevant to load gas on the field lines. In this work, we analyse whether warm disc winds can account for these UV observations.To get a good hint on the physics, we have preferred to focus on analytical work. We have made use of the warm disc wind solution with pressure calculated by Vlahakis et al. The rich complexity of the magnetohydrodynamic (MHD) disc wind kinematics is analysed in detail. We show the following. (i) Warm disc flows departing from the inner disc radius (∼0.1 au) reach terminal velocities close to the observed (300 km s −1 ) and that the temperature at the base of the wind is ∼40 000 K. Thus, the spectral signature of warm disc winds is an enhancement of the chromospheric and transition region spectral indicators by several orders of magnitude, as observed in cTTSs. (ii) Warm disc winds rotate, but their rotation velocity is small since the wind dynamics is based in a transference of angular momentum from the star to the magnetic field. Henceforth, slow rotation plus high temperature (e.g. rotational broadening) show in very broad profiles at the base of the wind. (iii) The kinematics at the base of the wind is dominated by axial expansion; however, above z 5 au, the dominant kinematical component is acceleration along the disc axis. (iv) The most often observed profile is a single-peaked line, slightly blueshifted and asymmetric with a tail to short wavelengths. The full width at half-maximum is clearly suprathermal and depends on the spectral tracer used.Line profiles, fluxes and line ratios are calculated for the C IV[uv1], C III] 1908 , C II] 2326 , Si III] 1892 and [O II] 2471 UV lines. In edge-on systems, the profiles range from symmetric lines with suprathermal broadenings for the spectral tracers of the base of the wind (C IV[uv1] and Si III] 1892 ) to double-peaked profiles for spectral tracers more weighted to higher values of z ([O II] and C II]). In pole-on systems, the profiles are blueshifted (from few to 200 km s −1 ) and asymmetric exhibiting long tails towards the terminal velocity of the wind (V rad = 300 km s −1 ). To reproduce the observed line fluxes and ratios, winds must be clumpy and extinction by the flaring disc and the wind itself ought to be taken into account.

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Section: Discussionmentioning

confidence: 99%

On the source of dense outflows from T Tauri stars - II. Warm disc winds

Castro

Ferro-Fontán

2005

Monthly Notices of the Royal Astronomical Society

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“…Since we study a forming current sheet where reconnection is just beginning,x magnetic components resulting from reconnected field do not yet exist. However, it is possible that the background magnetic field from which the sheet forms is curved; indeed this is the case in the Earth's magnetotail (Petrukovich et al 2015), and in astrophysical situations where current sheet formation occurs because of field-line opening due to differential rotation (see, e.g., Uzdensky (2004) and references therein). An extension of our work could consider such curved fields, perhaps by employing computational methods.…”

Section: Conclusion: Effect Of Flow In the Context Of Current Sheet Dmentioning

confidence: 99%

“…2015), and in astrophysical situations where current sheet formation occurs because of field-line opening due to differential rotation (see, e.g. Uzdensky (2004) and references therein). An extension of our work could consider such curved fields, perhaps by employing computational methods.…”

Section: Conclusion: Effect Of Flow In the Context Of Current Sheet Dmentioning

confidence: 99%

Development of tearing instability in a current sheet forming by sheared incompressible flow

2018

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Sweet-Parker current sheets in high Lundquist number plasmas are unstable to tearing, suggesting they will not form in physical systems. Understanding magnetic reconnection thus requires study of the stability of a current sheet as it forms. Formation can occur due to sheared, sub-Alfvénic incompressible flows which narrow the sheet. Standard tearing theory (Furth et al. 1963;Coppi et al. 1976;Rutherford 1973) is not immediately applicable to such forming sheets for two reasons: first, because the flow introduces terms not present in the standard calculation; second, because the changing equilibrium introduces time dependence to terms which are constant in the standard calculation, complicating the formulation of an eigenvalue problem. This paper adapts standard tearing mode analysis to confront these challenges. In an initial phase when any perturbations are primarily governed by ideal MHD, a coordinate transformation reveals that the flow compresses and stretches perturbations. A multiple scale formulation describes how linear tearing mode theory (Furth et al. 1963;Coppi et al. 1976) can be applied to an equilibrium changing under flow, showing that the flow affects the separable exponential growth only implicitly, by making the standard scalings time-dependent. In the nonlinear Rutherford stage, the coordinate transformation shows that standard theory can be adapted by adding to the stationary rates time dependence and an additional term due to the strengthening equilibrium magnetic field. Overall, this understanding supports the use of flow-free scalings with slight modifications to study tearing in a forming sheet.

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Magnetic Interaction Between Stars and Accretion Disks

Abstract: Abstract.In this review I consider modern theoretical models of coupled star-disk magnetospheres. I discuss a number of models, both stationary and time-dependent, and examine what physical conditions govern the selection of a preferred model.

Cited by 2 publications

References 26 publications

On the source of dense outflows from T Tauri stars - II. Warm disc winds

On the source of dense outflows from T Tauri stars - II. Warm disc winds

Development of tearing instability in a current sheet forming by sheared incompressible flow

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